1. Field of the Invention
This invention relates to a circuit for automatically selecting an intermediate frequency. More particularly, it relates to a circuit which automatically selects and outputs an intermediate frequency suitable for a particular broadcasting system by providing multiple local oscillators, each of which are suitable for a specific broadcasting system.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a conventional television receiving apparatus. In FIG. 1, a high channel frequency is received by an antenna and is amplified by a high frequency amplifier portion 101 in a tuner 100 and is output through a mixer 103. The mixer 103 mixes the high channel frequency amplified from the high-frequency amplifier portion 101 with an oscillating frequency output from a local oscillator 105 and outputs the difference frequency of these signal frequencies. In other words, the output of the local oscillator 105 minus the output of the high frequency amplifier portion 101 equals the frequency of the output of the mixer 103, which is an intermediate frequency IF.
The main purpose of high frequency amplification is to prevent local oscillating power from leaking outside. The local oscillator 105 oscillates at a higher frequency than that of the image carrier wave frequency of the received channel. The local oscillating frequency is as high as an image intermediate frequency. The output of the mixer 103 is input to a Phase Lock Loop (PLL) Portion 107. The PLL 107 accurately synchronizes an intermediate frequency with a predetermined channel frequency and outputs it to an intermediate frequency amplifier 300 by means of serial clocks (SCLs) and serial data (SDA) output by a microprocessor 200. The intermediate frequency amplifier 300 amplifies the intermediate frequency and outputs it to an image processor 400 and a sound multiplex processor 500 in order to obtain image detecting waves and sound signals. The image processor 400 detects a luminance signal, a carrier wave color signal, and a synchronizing signal among the intermediate frequency signals amplified by the intermediate frequency amplifier 300. The sound multiplex processor 500 detects an FM sound signal by means of a 4.5 Mz sound detection circuit.
In FIG. 1, the intermediate frequencies are different from each other, and depend on a particular broadcasting system of each broadcasting station. To output an intermediate frequency suitable for a particular broadcasting system, a separate tuner for respectively outputting a different intermediate frequency needs to be developed for each broadcasting system corresponding to each appliance (Television, Video, VCR). Therefore, simplification of design is difficult and an export market is limited.
On the other hand, U.S. Pat. No. 5,014,349, TELEVISION TUNER FOR COMMON USE IN BC/UV, shows the technology to simplify the circuit and to reduce the expense of manufacturing the circuit by making common use of a double super type tuner which receives a TV signal in the VHF/UHF band and/or a CATV signal, and a BS tuner which receives a broadcast signal from a satellite TV by means of a switching circuit. In this prior art, when at least one of a VHF/UHF band TV signal and a CATV signal is input as a first input signal and a BS IF signal is input through the frequency conversion of the satellite TV broadcast signal as a second input signal, a tuner portion is made in the form of a double super type of an up/down system so as to receive the TV signal of the VHF/UHF band and/or the CATV signal.
The double super type tuner includes an up-converter comprising a first partial oscillator and a first mixer, and a down-converter comprising a second partial oscillator and a second mixer. After the satellite TV broadcast signal is input as the second input signal, the first partial oscillator and the first mixer are switched by a switching circuit and used as the down-converter. Therefore, making common use of the first partial oscillator and the first mixer for reception of the TV signal of the VHF/UHF band and/or the CATV signal and for reception of a satellite TV broadcast signal by the switching circuit results in simplifying the structure, cutting down expenses, and miniaturizing the receiver.
However, even though the technology makes common use of the receiving tuner for the TV signal of the VHF/UHF band and/or the CATV signal, and the satellite TV broadcast signal and results in simplifying the structure, reducing expenses, and miniaturizing the receiver, in order to output the intermediate frequency for a particular broadcasting system this technology still has a disadvantage in that a plurality of tuners are required.